How to perform multiplication, using bitwise operators?

Question

I am working through a problem which i was able to solve, all but for the last piece - i am not sure how can one do multiplication using bitwise operators:

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Answer 1

-(int)multiplyNumber:(int)num1 withNumber:(int)num2
{
    int mulResult =0;
    int ithBit;

    BOOL isNegativeSign = (num1<0 && num2>0) || (num1>0 && num2<0)   ;
    num1 = abs(num1);
    num2 = abs(num2);


    for(int i=0;i0){
            mulResult +=(num1<